Pulley and RPM

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Mar 2007
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MidWest
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## Pulley and RPM

I was wondering if anyone has an idea how I can calculate RPM from pulley size. Specifically, I want to mount an electric motor to an old cement mixer. The main wheel on the mixer is 64inches in circumference. I have several electric motors, most 1725 RPM, but I do have one around 1125. Rather than shoot in the dark, I'm hoping someone has a formula for calculating RPM on the main wheel based on a motor pulley size and RPM. The mixer wheel is on a 1 to 1 gear ratio with the mixer drum. Hope this makes sense. Thanks.

2. ## Re: Pulley and RPM

Mixer x: Circumference of Mixer Cx = 64 inches, RPMx = ?
Motor m: Circumeference of motor Cm = ?, RPMm = ?

Likely the formula you seek is: Cx X RPMx = Cm X RPMm
Thus RPMm = Cx X RPMx / Cm or RPMm = Cx/Cm X RPMx
In other words, the new rpm is the ratio of the pulley circumerferences x old RPM
Since Circumference = pi X D or 3.14159 X Diameter, you can use the ratio of pulley diameters in place of pulley circumferences:
RPMm = Dx/Dm X RPMx or.... RPMx = Dm/Dx X RPMm

Example
Your 64 inch circumference mixer pulley has a diamter of 64 /3.14159 = 20.4 inches
Suppose you put a 4 inch diameter pulley on a motor that turns at 1725 rpm.
The RPM of the mixer will be: RPMx = (Dm/Dx) X RPMm = (4/20.4) x 1725 = 338 rpm
You can figure the rest of the variations from there.

Join Date
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Location
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## Re: Pulley and RPM

Originally Posted by Rick V

Example Your 64 inch circumference mixer pulley has a diamter of 64 /3.14159 = 20.4 inches

Correct, I measured the diameter but neglected to quote the size in the OP.

Suppose you put a 4 inch diameter pulley on a motor that turns at 1725 rpm.
The RPM of the mixer will be: RPMx = (Dm/Dx) X RPMm = (4/20.4) x 1725 = 338 rpm
You can figure the rest of the variations from there.
Thanks Rick, yes I will take it from here. Not exactly sure what the industry standard is for mixer drum RPMs, but my instincts tell me 300 would be too fast. So I will see what diameter pulley I can get for the motor and probably the 1125 will be best suited.
Thanks again for the prompt reply, I will save this.

4. ## Re: Pulley and RPM

Most mixers go about 20 rpm. Larger mixers may go even slower. The last motor I used was a 1725. The motor will have a small 1.5" or 2" pulley. The drive gear should have a largish pulley, maybe 12" or better, never measured the pulleys, just swapped motors. From your description it wasn't clear if you had the large pulley which is why the rpm was so high.

Did you get a ratio on the gear box to the drum ring? Turn the drum and count the revolutions on the gear box, you need to know this so you can calculate the required input rpm. You can play with both pulley sizes until you get the desired rpm. This page may help ounce you know your required input rpm (drum/gearbox ratio).
http://www.culvermotor.com/Engineeri...alculator.html

I found this picture on the web that shows the common design.
Last edited by forhire; 06-27-2010 at 03:52 AM.

Join Date
Mar 2007
Location
MidWest
Posts
402

## Re: Pulley and RPM

Originally Posted by forhire
Most mixers go about 20 rpm. Larger mixers may go even slower. The last motor I used was a 1725. The motor will have a small 1.5" or 2" pulley. The drive gear should have a largish pulley, maybe 12" or better, never measured the pulleys, just swapped motors. From your description it wasn't clear if you had the large pulley which is why the rpm was so high.

Did you get a ratio on the gear box to the drum ring? Turn the drum and count the revolutions on the gear box, you need to know this so you can calculate the required input rpm. You can play with both pulley sizes until you get the desired rpm. This page may help ounce you know your required input rpm (drum/gearbox ratio).
http://www.culvermotor.com/Engineeri...alculator.html

I found this picture on the web that shows the common design.
forhire, thanks for the info. I'll check the www site out later today. I think I am well on my way now getting my mixer operational. I'll probably pick up a small pulley as you quoted. No, I have not looked at the gearbox to drum ring ratio but will do either later today or Monday. I need to factor that in. Thanks again, will advise how this turns out.

Join Date
Mar 2007
Location
MidWest
Posts
402

## Re: Pulley and RPM

Rick/forhire,

7 revolutions of the CM large wheel (20 inches diameter) to 1 revolution of the CM drum. Based on formula Rick provided, 2 inch (motor pulley diameter) / 20 inch (CM pulley diameter) X 1725 = 172 RPM for the CM large wheel. Divide that by 7 = 24 RPM CM drum. That should work. So I am off tomorrow to find a 2 inch pulley for my 220 VAC 1725 motor, Thanks guys.

My CM is nearly exact to the pic forhire posted.
Last edited by geezer; 06-27-2010 at 09:15 AM. Reason: content

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